[vlc-devel] a few questions about module behaviour

[Laurent Aimar]

Hi,
On Tue, Sep 25, 2007, Tomas Carnecky wrote:
> The video resolution or sound format of the input stream can change at
> any time. I have taken care of the video problem by making the output ES
> fixed size and allocating a canvas of that size. When an input frame
> comes in, it is copied into the canvas, and than the canvas is
> duplicated into the output ES. That works reasonably well, I'll have to
> figure out something for the changing sound format (but that's less of
> an issue right now).
 You can add and delete ES at the runtime, and so every time the video/audio
properties change.
 Then you may use the sout module transcode and gather like this:
 transcode{vcodec=I420,with=720,height=576,acodec=s16l,samplerate=44100,channels=2}:gather:your_sout_part
 to be sure to create a video/audio ES that never change.
(It might be a bit tricky if you have more than one audio or one video)

> It sort-of works if I send out only video frames, but once I add audio
> vlc goes crazy and complains about bad timing, like:
> [...] audio drift is too big (236985), dropping buffer
> [...] late picture skipped (2240)
> 
> Once thing that really amazed me is when I changed Demux() to only do:
>   copy = block_Duplicate(canvas)
>   copy->pts = date_Increment(video.pts, 1)
>   es_out_send(copy)
>   return 1;
> I still saw 'late picture skipped' messages. I don't understand, I can
> hardly push out frames faster than that, or is there a scheduling
> problem somewhere?
 You have to call es_out_Control( p_demux->out, ES_OUT_SET_PCR, pcr )
regularly (for example every time you call es_out_Send on video data.
 The value pcr is used to regulate the way VLC eats data. I think the right
value in your case can simply be the video timestamp.
 The value you return on DEMUX_CAN_CONTROL_PACE (p_demux->pf_control) is also
important :
 - VLC_TRUE: it means that your source use the same clock than the PC vlc is
used on (as you use gettimeofday, I think it is the right choice)
 - VLC_TRUE: your source use a non synchronized clock,
 es_out_Control( ES_OUT_SET_PCR ) will be use to correct the drift between
the distant clock and the PC one.
(And don't forget to return a sensible value on DEMUX_GET_PTS_DELAY, like 300ms)

 Regards,

-- 
fenrir